Flooding correlation

Fig. 23. Pressure drop and flooding correlation for various random packings (95). ip = p- o IP-l (standard acceleration of free fall) = 9.81 m/s, p, = liquid viscosity ia mPa-s numbers on lines represent pressure drop, mm H2O /m of packed height to convert to ia. H2O /ft multiply by 0.012. Packing...

Fig. 18. Flooding correlation for crossflow trays (sieve, valve, bubble-cap) where the numbers represent tray spacing in mm. Also shown are approximate...

FIG. 14-25 Flooding correlation for columns with crossflow plates (sieve, valve, bubble-cap). [Fair, Pet/Chem Eng 33(10), 45 (September 1961),]... 

For distillations, it is often of more interest to ascertain the effect of entrainment on efficiency than to predic t the quantitative amount of liquid entrained. For this purpose, the correlation shown in Fig. 14-26 is useful. The parametric curves in the figure represent approach to the entrainment flood point as measured or as predicted by Fig. 14-25 or some other flood correlation. The abscissa values are those of the flow parameter discussed earher. The ordinate values y are fractions of gross hquid downflow, defined as follows ... 

FIG. 14-49 Pressure drop/flooding correlation of Kister and GiU for 2-inch metal Pali rings. The upper chart is for nonaqueous systems, the lower chart for aqueous systems. To convert inches H20/ft to mm H20/m, multiply by 83.31. 

Flooding of Agitator Impellers A review of impeller flooding has been done by Sensei et al. [AlChE Symp. Series No. 283, 89 (1993)] and they have offered the following flooding correlation for a six-bladed disk-type turbine. 

DEL = Column flooding correlating factor FPL = Tray flow path length, ins. 

Recommended Range of Application. The Kister and Haas [184] Flood Correlation (used by permission, Kister, H. Z., Distillation Design, McOraw-HiU, Inc., 1992)... 

Flooding correlation (no tapered inlet tube considered) ... 

English s flooding correlation incorporates an entrainment load of E/G from 0.01-0.05 lb liquid per lb of vapor. 

Figure 18.5. Entrainment from sieve trays in the units mols liquid entrained/mol of liquid downflow LM, is the weight rate of flow of liquid and VMv is the weight rate of flow of vapor. The flooding correlation is Figure 13.32(b). [Fair and Matthews, Pet. Refiner 37(4), 153 (195S)].

The Souders and Brown constant CSB is the C-factor [Eq. (14-77)] at the entrainment flood point. Most modern entrainment flooding correlations retain the Souders and Brown equation (14-80) as the basis, but depart from the notion that CSB is a constant. Instead, they express CSb as a weak function of several variables, which differ from one correlation to another. Depending on the correlation, CSB and us,flood are based on either the net area or on the bubbling area AB. 

For low-capacity random packings, such as the small first-generation packings and those smaller than 1-in diameter (Fp > 60 ft-1), calculated flood pressure drops are well in excess of the upper pressure drop curve in Fig. 14-55. For these packings only, the original Eckert flood correlation Chem. Eng. Prog. 66(3), 39 (1970)] found in pre-1997 editions of this handbook and other major distillation texts is suitable. 

Flooding of Agitator Impellers Impeller flooding correlations for six-blade disk (6BD) Rusthon turbines and six-blade disk Smith turbines (Chemineer designation CD-6) are presented by Bakker, Myers, and Smith [Chem. Eng., 101, 98 (Dec. 1994)] and a review of impeller flooding. The Bakker et al. (loc. cit.) correlation is... 

Rotating packed-bed devices handle high volumes of fluids in a small equipment volume, compared to packed towers, due to the acceleration of gravity. The Sherwood flooding correlation for packed towers (25) is expressed as a plot of... 

Early RPB researchers discovered that this flooding correlation for packed towers applied equally well to RPBs when the gravity term (g) was replaced by centrifugal acceleration (ra>2). As acceleration increases, the gas flooding velocity (Uq) increases in order to maintain the same value of the first term. Since the ratio of liquid (L) to gas (G) flow remains constant, liquid flow increases commensur-ately with gas flow. Most researchers observed higher gas velocities before the onset of flooding than predicted by the Sherwood correlation (17,26,27). 

Tray area is usually determined from an entrainment flooding correlation. Trays are normally designed to operate at 80 to 85% of flood at the maximum expected throughput. Downcomer area is usually determined from the downcomer choke criteria. The design is then checked to ensure that downcomer backup flood does not occur. 

FIGURE 20 Entrainment flooding correlation for trays. (From Fair, J. R. (1961). Petrol Chem. Engineer Sept., p. 45 reproduced by permission of Petroleum Engineer International, Dallas, Texas.)... 

Liquid load Figure 6.9a is a plot of flood F-factor against liquid load. From Eq. (6,5), it also illustrates the effect of liquid load on the flood C-factor, or CSB. As liquid load increases, CSB first rises, then declines. The decline is slow, as illustrated by the cyclohexane-n-heptane curves in Fig. 6.9a, Some of the earlier flood correlations (18-20) predict that CSB rapidly diminishes at high liquid loads (as illustrated for the butane curves in Fig. 6,9a). Later work, first reported by Gerster et al. (21) and then by many others (15,22-26), however, showed that the rapid decline (such as that for the butane system) is associated with downcomer flooding, and not with entrainment flooding. 

CgB is practically independent of pressure in distillation systems (21,25 see also cyclohexane-n-heptane curves on Fig. 6.9a), This suggests that CSB is, at the most, only a very weak function of physical properties. Further, it suggests that any physical property variations that accompany a change in distillation pressure (and, therefore, also in the equilibrium temperature), do not affect CSB. This contradicts trends predicted by some of the earlier flood correlations (18-20). 

Figure 6.11 The Smith. Dresser, and Ohlswager entrainment flooding correlation. (R- B. Smith, T. Dresser and S. Ohlswager, Hydmcarb. Proc. Pet. Ref., 40 (5), p. 183, 1963, reprinted courtesy of Hydrocarbon Processing.)...

The restrictions applying to the use of Fair s flooding correlation (Sec. 6.2,6) also apply to the use of Fair s entrainment correlation (5). 

Tray area. Using an entrainment flooding correlation is the usual method of calculation, but short-cut methods (e.g., Ref. 10) are often adequate. However, to minimize trial and error, the author prefers to perform the preliminary estimate using the flooding correlation which will eventually be used, with some simplifying assumptions. In this example, the Kister and Haas correlation [Eq. (6.12)] will be used for the preliminary determination of tray area. Make the following simplifying assumptions ... 

The example reflects the prime difficulty often encountered by tray designers inconsistent predictions from different correlations. The three entrainment flood correlations used gave predictions that widely differed the differences were up to 50 to 60 percent- Another inconsistency was in the weep-dump prediction. These inconsistencias stem from the empiricism associated with prediction methods. Our understanding of tray hydrodynamics has still a very long way to go before it can provide us with models that reliably predict tray performance from first principles. 

Based on the above reliability studies, the author recommends the Eckert flood correlation (the flood curve on Fig. 8.17) only for random packings whose packing factors Fp exceed 60. 

Mackowiak (73a, 736) derived a new flood correlation. Like the Billet and Schultes correlation, it is based on the drop entrainment modal and takes liquid holdup into account. Unlike Billet and Schultes, Mackowiak uses a different set of premises and expressions. Ma6ko-wiak s correlation applies for both random and structured packings, has a good theoretical basis and was shown (73a, 786) to predict a large number of flood data to within 8 percent. On the debit side, the correlation is complex and requires the availability of four constants for each packing. Matkowiak also states (73a) that for high liquid rates, Mersmann s film model is more suitable than his drop model. 

Others, Flood correlations are often available in the manufacturer literature (e.g., 8,31,82) or on manufacturers computer disks. A flood correlation by Beck (83) was shown by MacDougall (58) to be accurate, but applies only for Pall and Raschig rings and Berl and Intalox saddles. 

Fig. 23. Pressure drop and flooding correlation for various random packings (95). V = 0 /Pl> (standard acceleration of free fall) = 9.81 m/s2,...

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